The toothed portions of automotive steering racks are known to be produced by either a machining or a forging process. Typically, the machining process comprises broaching across a solid cylindrical bar resulting in the cross section of the toothed portion having a ‘D’ shape and hence these racks are commonly referred to as “D-racks”. Steering racks having machined teeth can only be economically mass produced with constant pitch teeth. However, racks having forged teeth can be equally mass produced with either constant or variable pitch teeth.
The term “flash” when used with respect to forging refers to the excess material that extends out from the body of a forged component and must typically be removed by a subsequent trimming or machining operation. Flash is a common feature of open die forging, in which case excess material is placed in the die to ensure complete filling of the die cavity. The term “flashless forging” refers to a forging process in which virtually no excess material is allowed to escape from the die cavity. The advantages of flashless forging include the elimination of waste material, elimination of subsequent operations to remove the flash, and greater control over the precision of the forged component. Flashless forging is typically achieved by using a closed forging die.
U.S. Pat. No. 4,571,982 (Bishop) and U.S. Pat. No. 5,862,701 (Bishop et al) disclose a die apparatus for flashless warm forging the toothed portion of a steering rack to net shape from a solid cylindrical bar. “Net shape” means that the forged rack teeth do not require any further machining after forging. This type of die apparatus is limited to forging racks where the cross section of the toothed portion has a ‘Y’ shape and such racks are commonly referred to as “Y-racks”. It is important to note that this type of die apparatus only forms a closed forging cavity at the end of the forging process as can clearly be seen from FIGS. 7, 8 and 9 of U.S. Pat. No. 4,571,982, and the substantially flashless forging results from the unique motion of the tooling elements and the ‘Y’ shaped cross section of the toothed region, rather than closing the forging cavity prior to the completion of the forging operation. A disadvantage of Y-racks is that they require modification of the steering gear to enable assembly and as such the market has historically preferred D-racks.
Various types of die apparatus have been proposed for forging D-racks from solid bar. However, most of these dies produce flash. FIGS. 3 to 5 of JP 58218339 (Daido Steel Co Ltd) depict a basic open die apparatus comprising only two die halves. The excess material simply escapes the die cavity as flash. This die apparatus provides no means for controlling the cavity pressure and as such the resulting tooth fill is likely to be poor, particularly if the forging operation is performed at warm forging temperatures rather than hot forging. FIG. 5 of JP 58218339 illustrates the process of trimming the flash after forging.
GB 2108026 (Cam Gears Ltd) discloses a die apparatus for forging a D-rack from a solid bar. This is a basic die apparatus having two halves with the addition of flash gutters directed at controlling the formation of the flash and assisting tooth fill. However, material may still escape into these flash gutters prematurely, thereby limiting the admitted hydrostatic pressure which may cause under filling of the die cavity. The flash produced by such a die apparatus is more controlled in shape than that produced by a simple open die but the flash would still typically need to be removed after forging. A more sophisticated die apparatus using a similar principle is disclosed in U.S. Pat. No. 5,992,205 (Bishop), which is directed at shaping the flash gutters to maintain adequate hydrostatic pressure and thereby assist in achieving an adequate tooth fill.
Die apparatus for forging solid D-racks are disclosed in JP 58013431 (Jidosha Kiki Co Ltd) and JP 03138042 (IS Seiki KK et al). Both these dies forge resulting toothed portions that are greater in enclosing diameter than the shank of the finished rack, which is usually the nominal diameter of the bar stock. Such racks have the same assembly problems as Y-racks. Furthermore, both these dies only comprise two halves and as such are unlikely in practice to fully close and fill the teeth without flash forming between the die halves.
FIGS. 8 to 12 of JP 58218339 (Daido Steel Co Ltd) depict a closed die apparatus for forging a steering rack from hollow tube. This die closes before forging starts and therefore the forging process would be substantially flashless. The punch 56 of this die arrangement has a shape that forges the teeth of the rack. Punch 56 moves inwards to perform the forging operation after die halves 58 and 50 close around the hollow tubular blank. The closing of die halves 58 and 50 does not cause any deformation of the blank. The problem with this die arrangement is that the ends of the teeth on punch 56 are open and as such there is no support between the ends of the teeth, which could lead to premature die failure. Furthermore, the ends of the forged teeth are perpendicular to the teeth, rather than sloping at each end, which could interfere with assembly of the rack.
It is an object of the present invention to provide a die apparatus and method for flashless forging of steering racks that ameliorates at least some of the problems of the prior art.